With the critical dimensions (CDs) of photolithography patterns shrinking to 22 nm or less, especially with the increasingly extensive use of double patterning techniques, overlay measurement accuracy, as one metric measuring the performance of photolithography processes, is required to be on the order of sub-nanometers. Due to imaging resolution limits, traditional overlay measurement techniques based on imaging and image recognition (i.e., imaging-based overlay (IBO) techniques) have been increasingly important to meet the overlay measurement requirements of new technical nodes. Currently, overlay measurement techniques based on diffracted light detection (i.e., diffraction-based overlay (DBO) techniques) are increasingly prevalent in the field of overlay measurement. The greatest challenge faced by the existing DBO techniques is that the marks, which are large, take up an excessively large part of the effective exposure area, leading to excessive cost of the marks. In addition, in order to be in line with the overlay measurement requirements of new technical nodes, it is necessary to carry out the overlay measurement in the exposure field. However, large marks are not suitable for in-field measurement. Thus, scaling-down of overlay marks is an inevitable trend in the development of DBO techniques.
A DBO technique proposed in the prior art is to obtain an overlay error through measuring asymmetry between diffracted light components of the same order of diffraction in angular resolution spectra for an overlay mark. The angle at which incident light is diffracted varies with the incident light, and the so-called angular resolution spectra of diffracted light refer to strength distributions of light diffracted by the mark at different angles when light is incident on the mark at different angles. FIG. 1a shows an angular resolution spectra for different orders of diffraction (−2, −1, 0, 1, 2) formed on a CCD detector under an annular illumination condition. FIG. 1b is a structural schematic of an apparatus for this technical solution. Light emitted from a light source 2 is focused by a lens L2 and then shaped by an interference filter 30 into an incident light beam of a narrow bandwidth. An objective L1 then condenses the incident light onto an overlay mark in a substrate 6, wherein the overlay mark is typically consisted of two stacked linear gratings. In the figure, F denotes a focal length of the objective. An overlay mark detector 32 is arranged on a rear focal plane 40 of the objective L1, and diffracted light from the overlay mark is collected at the objective L1 and then reflected by a reflector surface 34 onto and received by the overlay mark detector 32. The overlay mark detector 32 measures angular resolution spectra of diffracted light from the overlay mark at different angles. In order to obtain angular resolution spectra over a large range, an objective with a large numerical aperture (NA) is adopted in the solution. As known from the above description, first, according to its measuring principles, the overlay mark has to assume a large size. In addition, it is not feasible to reduce the mark size through decreasing the size or number of pitches of the gratings. This is because with smaller pitches in the gratings, diffracted light of higher orders may become evanescent and cannot be collected, leading to non-detection of corresponding overlay signals, and when the number of pitches in the overlay mark is reduced to a certain value, diffracted light components of different orders will no longer strictly follow the grating diffraction equation, disabling the overlay error calculation based on the detected diffracted light signals. Therefore, this solution is incapable of in-field measurement using small marks. Additionally, in this solution, overlay information is obtained based on the detection of light strength signals for diffracted light, making the overlay measurement accuracy susceptible to system illumination uniformity and transmission uniformity.